1. Field of Disclosure
This disclosure relates generally to 3D silicon stacking, and in particular to circuits constructed on multiple dies stacked into tiers.
2. Background
One feature of through silicon or 3D stacking technology is the construction of a complete circuit on multiple dies stacked into tiers. Each part of the circuit is prepared on a separate die using conventional Si processing, then the dies are stacked together and joined to create a complete circuit. By bringing various parts of a circuit closer together and reducing chip footprint size, through silicon stacking can increase chip speed and decrease the size of a semiconductor package.
The way that 3D stacking technology can be incorporated into circuits ranges from the placement of different circuit functions in separate tiers, to the formation of a single logic gate split across multiple tiers. At an intermediate level, a single functional unit can be divided over multiple tiers. For example, an arithmetic logic unit (ALU) can have its computation and register portions on separate tiers, providing shorter path lengths and increased speed between the computational and register functions.
Unless a die fabrication process yields a very high percentage of good dies, some form of screening or testing of individual dies prior to their incorporation into a multiple tier stack is advantageous. Otherwise, the loss of stacks due to the incorporation of a defective die can become significant. By its very nature, however, 3D stacking technology involves dies having tier-to-tier connections that are difficult to test by conventional methods before the dies are incorporated into a stack. The difficulty is that the circuits associated with the tier-to-tier connections are logically incomplete until the dies are connected. Thus, a method of testing a die having such logically incomplete circuits is desirable.